Many types of mechanical and electrical operative equipment contain a dielectric fluid for dissipating heat generated by energized components, and/or for insulating components from one another or from the operative equipment enclosure. Examples of such operative equipment include various types of electrical equipment such as transformers, capacitors, switches, and load tap changers. Dielectric fluids used in operative equipment typically comprise blends of compounds in the group including: paraffinic, naphthenic, and other organic fluids such as polyalphaolefins, esters, and natural vegetable oils.
Large electrical transformers generally rely upon dielectric fluids such as transformer oil, for the purpose of cooling and insulating transformer components. Over time, components within the transformer, such as insulating paper or the dielectric fluid itself, degrade. Also, if the transformer overheats or malfunctions, the dielectric fluid may further degrade. These degrading conditions cause contaminants such as gases, moisture and other volatile compounds to be diffused into the dielectric fluid. The volatile compounds may include a host of products, including products originating from the decomposition of insulating paper or its additives. As a result, the dielectric fluid's insulating and cooling properties are altered, thereby diminishing the transformer's efficiency and promoting transformer failure. In general, the dielectric fluid's and solid insulation's properties degrade more rapidly in the presence of dissolved moisture and oxygen, which may ingress through gaskets, seals or oil expansion systems from the atmosphere or as a result of solid decomposition.
To maintain ideal dielectric fluid properties, any impurities present in a dielectric fluid need to be removed periodically. Typically, removal of dissolved gases, moisture and volatile compounds from dielectric fluid is accomplished by applying a combination of a vacuum and heat to the dielectric fluid. Applying a vacuum to the dielectric fluid while a transformer is in use can generate bubbles and put the transformer at risk by compromising its dielectric integrity. A transformer therefore is usually removed from service in order to purify the dielectric fluid. In most cases, the oil is degassed, dehydrated, and purified using a combination of vacuum and fuller's earth processes in external oil processing equipment and by recirculating the oil within the transformer tank.
Separation of dissolved gases from insulating oils has been accomplished in the prior art by permeation through thin wall tubing. This has however proved to be too slow for on-line degassing, moisture removal or near real time monitoring of dissolved gases in transformers oils.
U.S. Pat. No. 4,437,082 discloses a means of continually upgrading transformer oil by passing the oil through a series of filters. A degassing filter uses a permeable membrane of polymeric material to filter low molecular weight gases from the oil. This design utilizes a flat membrane structure which does not remove a satisfactory amount of gas from the oil because the surface area of oil exposed to the gas removing membrane is not optimal.
U.S. Pat. No. 5,914,154 discloses a process for making a gas permeable membrane and filter for purifying a gas. A gas containing impurities, such as oil droplets, is purified by trapping the impurities on one side of the membrane while recovering the desired purified gas from the other side of the membrane. This apparatus does not address degassification of fluids.
There are several products commercially available for degassing water or removal of organic compounds from water using porous hollow fibers. These products, however, are not compatible with insulating oils. U.S. Pat. No. 4,986,837 discloses an apparatus for degassing a liquid such as water or the like, using a semi-porous filter. This apparatus is however unsuitable for degassing dielectric fluids.
It is well known in the art to monitor the contents of dielectric fluid by sampling of gas or dielectric fluids for laboratory analysis. If the contaminants present within the dielectric fluid could be separated from the fluid and independently analyzed, such an analysis would lead to a more accurate determination of the overall condition of the transformer. Such an analysis would also facilitate in situ analysis and continuous on-line monitoring of gases, moisture and volatile compounds indicative of incipient dielectric failure or excessive overheating of insulating components.
In view of the foregoing, it would be highly desirable to provide an apparatus and method for continuously monitoring and purifying a dielectric fluid while it is in use by operative equipment.